What I believe what Patek is showing with the use od coloured sunburst dials is their expertise in dial making.
As far as I know from jewellery making the colours blue, brown, grey and even green are shades that can be created from gold.
Even purple can be realised from gold!
I do not know for sure if the dial of a coloured Patek is made out of gold but would expect it to be.
Unfortunately Patek is not saying in their descriptions whether the dial is out of gold, but I recall an article saying that If I am not mistaken.
Maybe you or somebody else know it?
Best
Moritz